1. Field of the Invention
The present invention relates to a method and an apparatus for depositing a silicon oxide film. In particular, the invention is applicable to a film deposition apparatus wherein a plasma generation chamber is divided from a deposition chamber, radicals are extracted from the plasma generation chamber to the deposition chamber, and a thin silicon oxide film is deposited on a substrate by the radicals being made to react with a process gas including silicon atoms supplied to the deposition chamber. In this deposition apparatus, the supply rate of the process gas in an initial stage after deposition is started, that is, in a first half side time constituting not more than half of the whole film deposition time, is limited.
2. Description of the Related Art
There have been deposition apparatuses wherein a deposition chamber and a plasma generation chamber are spatially divided and the deposition chamber and the plasma generation chamber each form a separate space. The divided structure of the deposition chamber and the plasma generation chamber in this kind of deposition apparatus prevents the plasma from making contact with the substrate. Only a gas for plasma generation is fed to the plasma generation chamber, and there a plasma discharge occurs under predetermined conditions and neutral active radicals are created. A process gas for film deposition is fed to the deposition chamber, and the above-mentioned radicals are also supplied to the deposition chamber through multiple holes formed in a partition disposed between the deposition chamber and the plasma generation chamber. Film deposition on a substrate in the deposition chamber is based on CVD (Chemical Vapor Deposition) resulting from a reaction between the radicals and the process gas. In the deposition of a silicon oxide film using this CVD method, a gas including silicon atoms is used as the process gas. The process gas is mixed with the radicals in the deposition chamber. As necessary, a carrier gas is also introduced. The process gas including silicon atoms is for example a silicon-hydrogen compound.
In related art methods for depositing a silicon oxide film, the flow of the process gas fed to the deposition chamber has normally been so controlled that from the start of plasma discharge to the end of plasma discharge (i.e. for the whole film deposition time) the flow is held to a required constant value determined by deposition conditions. Also, in the silicon oxide film deposition method discussed above, to raise the deposition rate to improve productivity and practicality, the flow of the process gas fed to the deposition chamber in the film deposition immediately following the start of plasma discharge has been set to a relatively high value.
However, as another side to this, when the supply rate at which the process gas is supplied in the film deposition immediately following the start of plasma discharge is too great, a silicon oxide film containing excess silicon is deposited. Silicon in the silicon oxide film produces an effect of carrier trap levels. Consequently, with a silicon oxide film including excess silicon, because a great many carrier trap levels are formed in the film, its electrical characteristics are poor. And when a silicon oxide film including excess silicon is used in a semiconductor device, the device characteristics deteriorate markedly.
Specifically, when a silicon oxide film including excess silicon is used as a gate insulating film of a TFT, the problem arises that the operating characteristics of the TFT fluctuate.
Considering this problem, to make good the device characteristics in the formation of the silicon oxide film, the flow of silicon atoms at the time of the plasma discharge should be set to a low value. In fact, J. Batey et al. have proposed in an article of theirs that to improve the electrical characteristics of a silicon oxide film it is effective to make the deposition rate of the silicon oxide film low (J. Appl. Phys. 60(9), Nov. 1, 1986). This article states as a conclusion that it is not possible to form a silicon oxide film having good electrical characteristics unless the film deposition rate is set to 0.13 nm/sec or less. However, because when the deposition rate is made 0.13 nm/sec or less like this the formation of the film takes time, from the point of view of practicality it is difficult to adopt this low deposition rate.
It is therefore an object of the present invention to provide a method and an apparatus with which it is possible to deposit a silicon oxide film having good electrical characteristics and which are effective from the practical application point of view.
A silicon oxide film deposition method provided by the invention is a method for forming a silicon oxide film on a substrate by generating a plasma in a plasma generation chamber divided from a deposition chamber, extracting radicals in the plasma from the plasma generation chamber to the deposition chamber, and causing these radicals to react in the deposition chamber with a process gas including silicon atoms fed to the deposition chamber. In this film deposition method, the supply rate of the process gas is, in a first half side time constituting not more than half of the whole film deposition time, first limited to zero or another desirably low supply rate and then controlled to gradually increase. The process gas supply rate in the first half side time can also be limited so that the thickness of film formed by the deposition in the first half side time is not greater than 10% of the overall thickness of the silicon oxide film.
In this method for depositing a silicon oxide film, a silicon-hydrogen compound (SinH2n+2 (n=1, 2, 3, . . . )) is preferably used as the process gas. An inert gas (a noble gas such as Ar) can also be introduced as a diluting gas along with the silicon-hydrogen compound process gas. In this case, the mixture proportions of the process gas and the inert gas can be determined freely.
And in this method for depositing a silicon oxide film, as the pattern of control of the increase of the process gas supply rate, preferably the increase is controlled in correspondence with time or is controlled on the basis of any of a linear function, a second-order function, an exponential function or a step function.
The rate at which the silicon oxide film is deposited in the first half side time constituting not more than half of the whole film deposition time is preferably not greater than 0.13 nm/sec.
As the gas for producing the radicals, any gas from among O2, O3, N2O, CO, CO2 and nitrogen oxide gases is used.
In a silicon oxide film deposition apparatus according to the invention, radicals are extracted to a deposition chamber from a plasma generation chamber divided from the deposition chamber, and in the deposition chamber the radicals are caused to react with a process gas including silicon atoms to deposit a silicon oxide film on a substrate. A feed part for feeding the process gas to the deposition chamber is provided between a process gas supply and the deposition chamber, and a mass flow controller (MFC) is provided in the feed part. The mass flow controller regulates the flow of the process gas supplied to the deposition chamber. A value determining the flow of the process gas is set in the mass flow controller. A host controller for issuing instructions to the mass flow controller dictating this set value determining the flow of the process gas is also provided. This host controller controls the process gas supply device rate by way of the mass flow controller. That is, the host controller, in a first half side time constituting not more than half of the whole film deposition time, first sets to zero or limits the process gas supply rate and then gradually increases it. The host controller may also limit the process gas supply rate in the first half side time so that the thickness of film deposited in the first half side time is not greater than 10% of the overall thickness of the silicon oxide film.
In this deposition apparatus, a silicon-hydrogen compound (SinH2n+2 (n=1, 2, 3, . . . )) is preferably used as the process gas. An inert gas (a noble gas such as Ar) can also be introduced as a diluting gas along with the silicon-hydrogen compound process gas. In this case, the mixture proportions of the process gas and the inert gas are freely determinable.
According to the invention, the feed rate of the process gas used for the deposition of the silicon oxide film is limited in a first half side time constituting not more than half of the whole film deposition time. The partial pressure of SiH4 or the like in the deposition chamber is lowered in the first half side time constituting not more than half of the whole film deposition time, and as a result the deposition rate of the silicon oxide film is suppressed and a state of excess silicon in the silicon oxide film is prevented. When a state of excess silicon is prevented, carrier trap levels in the silicon oxide film are reduced, and the film quality is improved. Also in the first half side time constituting not more than half of the whole film deposition time, the flow of the process gas is gradually increased on the basis of any of various patterns of control. By this means the deposition rate is increased, the overall time required for film deposition is shortened, the productivity of the film is raised, and the practicality of the deposition method or the deposition apparatus is raised. The supply rate of the process gas including silicon atoms is only limited for an initial period immediately following the start of plasma discharge, and thereafter the process gas supply rate is increased in any of various patterns of variation. When the formation of regions of SiO containing excess silicon in the silicon oxide film is prevented, the number of carrier trap levels is reduced, leak current is reduced and the electrical characteristics of the film are improved.